Hyunduck Cho

Bright and Efficient Full-Color Colloidal Quantum Dot Light-Emitting Diodes Using an Inverted Device Structure

We report highly bright and efficient inverted structure quantum dot (QD) based light-emitting diodes (QLEDs) by using solution-processed ZnO nanoparticles as the electron injection/transport layer and by optimizing energy levels with the organic hole transport layer. We have successfully demonstrated highly bright red, green, and blue QLEDs showing maximum luminances up to 23,040, 218,800, and 2250 cd/m(2), and external quantum efficiencies of 7.3, 5.8, and 1.7%, respectively. It is also noticeable that they showed turn-on voltages as low as the bandgap energy of each QD and long operational lifetime, mainly attributed to the direct exciton recombination within QDs through the inverted device structure. These results signify a remarkable progress in QLEDs and offer a practicable platform for the realization of QD-based full-color displays and lightings.

Liquid crystalline mesophases based on symmetric tetrathiafulvalene derivatives

A series of new symmetric tetrathiafulvalene derivatives were designed and synthesized. These compounds have highly ordered lamello-columnar liquid crystalline phases in addition to their crystalline phases at lower temperatures. Charge transport in the organic thin-film-transistor devices is faster in the liquid crystalline phase by two orders of magnitude than that in the crystalline phase due to the decreased defects.

Growth and characterization of thin Cu-phthalocyanine films on MgO(001) layer for organic light-emitting diodes

Surface morphology and thermal stability of Cu-phthalocyanine (CuPc) films grown on an epitaxially grown MgO(001) layer were investigated by using atomic force microscope and X-ray diffractometer. The (002) textured β phase of CuPc films were prepared at room temperature beyond the epitaxial MgO/Fe/MgO(001) buffer layer by the vacuum deposition technique. The CuPc structure remained stable even after post-annealing at 350°C for 1 h under vacuum, which is an important advantage of device fabrication. In order to improve the device performance, we investigated also current-voltage-luminescence characteristics for the new top-emitting organic light-emitting diodes with different thicknesses of CuPc layer.

Organic Thin-Film Transistors with Transfer-Printed Au Electrodes on Flexible Substrates

Bottom-gate, bottom-contact organic thin-film transistors (OTFTs) based on 6,13-bis(tri-isopropylsilylethynyl) pentacene (TIPS-pentacene) were fabricated on flexible substrates by transfer-printing and solution-based processes and their performances were characterized. Au gate and source/drain electrodes were transfer-printed from a release-layer-coated flexible mold at 2.4 MPa and 120 °C in ambient air. A polymer gate dielectric of poly(4-vinyl phenol) (PVP) and an active layer of TIPS-pentacene were deposited by solution processing. In addition, the TFT characteristics in channels of 40 and 4 µm length were compared. Also, the lack of saturation of the drain–source current (IDS) was investigated by considering the output characteristics in a short channel.

Effect of Electrode Area on High Speed Characteristics over 1 MHz of Poly(3-hexylthiophene-2,5-diyl) Diode with Inkjet-Printed Ag Electrode

We report the electrode-area dependence of the high-frequency characteristics of polymer diodes fabricated on a flexible poly(ether sulfone) (PES) substrate with inkjet-printed Ag electrode as an anode. Active layer of poly(3-hexylthiophene-2,5-diyl) (P3HT) layer was spin-coated on the Ag anode and then Al cathode was evaporated on the P3HT layer. The rectifying characteristics of P3HT diodes with different area sizes were compared, and their frequency response was analyzed for the sinusoidal input of different frequencies. The diode with smaller area exhibited higher breakdown voltage and operating frequency: The diode with a small area of 0.05 mm2 showed a 3 dB point at 1.8 MHz.

Charge transport in amorphous low bandgap conjugated polymer/fullerene films

The structural and charge transport properties of a low bandgap copolymer poly(3-hexylthiophene -alt-6,7-dimethyl-4,9-bis-(4-hexylthien-2yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline) (P(3HT-MeTDQ)) and its blend with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are investigated. Thermal analysis, X-ray scattering diffraction (XRD), atomic force microscopy and transmission electron microscopy (TEM) of P(3HT-MeTDQ) reveal that the polymer is amorphous in solid state. As the hole mobility of P(3HT-MeTDQ) was measured by the time-of-flight photoconductivity method, the mobility was 3.35 × 10−4 cm2/V s, which is very comparable to that of semicrystalline poly(3-hexyl thiophene). When the mobility of amorphous P(3HT-MeTDQ) was analyzed according to the Gaussian disorder model, the polymer has the energetic and positional disorders with the values of σ = 62 meV and Σ = 1.7, respectively, indicating that the polymer has a relatively narrow Gaussian distribution of transport states. Interestingly, when P(3HT-MeTD...

Characteristics of Inverters Using Pentacene Organic Thin Film Transistors with Printed Ag Electrodes

Inverters were fabricated with pentacene organic thin film transistors (OTFTs) using inkjet printing method. The gate, source and drain electrodes were printed with Ag ink. The mobility and threshold voltage of the bottom-contact OTFT were 0.078 cm2/Vs and 2.8 V, respectively. Printed inverters operated well in 200 Hz at VDD = −20 V.

38.4: Full‐Color Patterning of Quantum Dot (QD) Light‐Emitting Diodes using QD Transplanting Techniques

We provide simple and versatile QD transplanting method, which allows well-defined QD patterns down to a few micrometers and also provides compatibility with conventional device fabrication process and device architecture. We also demonstrate red, green, and blue QLEDs with transplanted QD active layers and characterize them in systematic correlation study between QD pattern morphology and device performances.

Multilayer white organic light emitting diode with optimum emitting layer sequence

Yellow organic light emitting diode (OLED) with red/green emitting layer (EML) sequence showed higher efficiency than device with green/red EML sequence because of the electrical carrier conduction difference in EMLs. Based on this result white OLED with additional blue emitter was demonstrated with maximum efficiency of 18.3 cd/A at 100 cd/m2 and good color stability.

Organic complementary inverter and ring oscillator on a flexible substrate

A complementary inverter was fabricated using pentacene and N-N′-dioctyl-3,4,9,10-perylene tetracarboxylic diimide-C8 (PTCDI-C8) for p- and n-type transistors on a poly(ether sulfone) substrate, respectively. The mobilities of the p- and n-type transistors were 0.056 and 0.013 cm2/V s, respectively. The inverter, which was composed of p- and n-type transistors, showed a gain of 48.6 when V DD=−40 V and at the maximum noise margin of V DD/2. A ring oscillator was also fabricated by cascading five inverters. The five-stage ring oscillator showed the maximum output frequency of 10 kHz when V DD=−170 V.